This invention relates to anaerobically curable coatings and adhesives, and more particularly to a process for anaerobic curing which uses compositions substantially free of known anaerobic initiators.
Anaerobic adhesives are liquid compositions stable in the presence of oxygen but curing or polymerizing upon exclusion of oxygen, generally by disposition between two substantially impervious surfaces. Such compositions are well known and are in substantial commercial use to lock or seal mechanical assemblies and as structural adhesives. They are described in numerous references, including for example, Sheist, "Handbook of Adhesives", pages 560-568, and U.S. Pat. Nos. 2,628,178; 2,895,950; 4,118,442; 4,103,081; 4,070,334; 4,056,670; 4,054,480; 4,044,044; 3,988,299; 3,933,748; 3,625,930; 3,435,012; 3,425,988; 3,300,547; 3,218,305; 3,203,941; and 3,046,262.
A wide variety of monomers and prepolymers useful in anaerobic polymerizing adhesives are known. Generally, they are acrylate monomers and terminally acrylated prepolymers. Anaerobic curing is initialed by catalysts which are diazonium compounds (see U.S. Pat. No. 4,070,334), sulfones (see U.S. Pat. No. 4,054,480) or peroxides, particularly organic peroxides, and most commonly hydroperoxides such as cumeme hydroperoxide. A variety of accelerators and inhibitors useful with such catalysts are also known. In the Sheist article cited above, at page 567, it is disclosed that some of the newer anaerobic adhesives may also be cured with exposure to ultraviolet light, a procedure described as useful in the bonding and sealing of glass.
Radiation curable coatings based on acrylates and acrylated prepolymers are also known and described, for example, in U.S. Pat. No. 3,989,609 and in references therein cited. For curing by exposure to ultraviolet radiation, photoinitiators or catalysts are used which are different from those used for anaerobic curing, for example, benzoin ethers. Such photoinitiators are described, for example, at lines 44-55, column 3, of U.S. Pat. No. 3,989,609. While it is known that oxygen can inhibit photopolymerization, anaerobic curing of such systems does not appear to have been previously recognized or described.